1. Field of the Invention
The invention relates generally to treatment of heart disease. More particularly, the invention is directed to an ultrasonic method and apparatus to treat ischemic tissue.
2. Description of the Background
Heart disease is a significant health problem and impairs the quality of life for millions of people. A common form of heart disease is ischemic heart disease, a condition in which parts of the heart muscle, or myocardium, do not receive an adequate supply of blood. Typically, this condition occurs when the arteries that carry blood to the myocardium become clogged by plaque build-up on their inner walls. The clogged arteries hinder blood flow, and the myocardium is deprived of oxygen and other nutrients. Ischemia results.
A number of methods are employed to improve blood flow to myocardium downstream of an arterial blockage. Many of these methods, such as coronary bypass surgery and balloon angioplasty, involve circumvention or removal of the arterial obstruction to re-establish blood flow. An alternate set of methods, known as transmyocardial revascularization (TMR) or percutaneous transmyocardial revascularization (PMR), involve the creation of small channels in the myocardium itself to reperfuse the ischemic tissue.
Channels created by the TMR or PMR procedures were initially believed to relieve ischemia by allowing blood to flow directly from the ventricle into the ischemic myocardium. More recent studies suggest that the channels do not remain open. Instead, the TMR or PMR procedures may stimulate angiogenesis, the creation of new blood vessels, and it is the new blood vessels that restore blood flow to the ischemic region. Angiogenesis is a natural response to cellular damage and results when injured cells alert the body to heal itself. It is believed that cells damaged by the TMR or PMR procedures produce and excrete special chemicals, such as cytokines and growth factors, which signal surrounding cells to initiate the formation of new blood vessels. The new blood vessels grow into the ischemic region, supplying the region with blood.
TMR and PMR methods used to create channels in the myocardium include mechanical coring, ultrasonic cutting, laser drilling, and using radio frequency (RF) energy to burn through the heart tissue. The mechanical, ultrasonic, laser, or RF device is typically positioned at the end of a catheter. The catheter is inserted either through the patient""s cardiovascular system to place the device into the inside of the heart or through a small cavity in the patient""s chest to place the device onto the outside of the heart.
Mechanical coring methods create channels in the myocardium by displacing or removing the heart tissue. Cutting devices such as needles or blades are employed.
Ultrasonic devices, such as those described in U.S. Pat. No. 5,827,203 to Nita and U.S. Pat. No. 5,989,274 to Davison et al., are also used to mechanically scrape or cut channels into the heart tissue. With these devices, ultrasonic energy is applied at frequencies between 20 kHz and 100 kHz to a tip at the end of a catheter. The ultrasonic energy causes the tip to vibrate and pierce the surface of the heart to form a channel. A blade may be attached to the tip to facilitate cutting.
Lasers, such as CO2 lasers, vaporize the heart tissue to burn channels in the myocardium. Myocardial revascularization using lasers is described, for example, in U.S. Pat. No. 6,074,384 to Brinkmann et al.
RF energy can also be used to burn holes in the myocardium, as described in U.S. Pat. No. 6,030,380 to Auth et al., U.S. Pat. No. 5,944,716 to Hektner, and U.S. Pat. No. 6,032,674 to Eggers et al.
A problem with the above procedures is that creating channels in the myocardium causes excessive trauma and damage to the heart tissue. The epicardium, endocardium, or both are punctured to form the channels, leading to a risk of complications such as hemorrhaging and scarring. The possibility that an embolus will form and cause, for instance, a stroke is another potential complication with the procedures.
As to problems with the particular methods described above, laser energy is known to kill healthy cells, which may worsen the patient""s condition. The laser procedure may also cause denervation, which relieves the chest pain associated with ischemia, but permanently damages the heart muscle. In addition, controlling the location and depth of a channel formed by laser or RF energy is difficult, making accidental damage to healthy tissue more likely. RF energy is also diffuse, making it especially difficult to localize damage from the RF energy device, and creating problems such as the coagulation of surrounding blood.
U.S. Pat. No. 5,827,203 also describes using low frequency ultrasonic energy to massage the ischemic myocardium, without cutting or removing the tissue, as is required when creating channels. However, although massaging the tissue is less traumatic to the heart tissue than creating channels, massaging alone does not fully treat ischemia and does not cause the cellular damage necessary to stimulate angiogenesis.
Embodiments of the present invention include methods and apparatuses for treating ischemic myocardium. The invention minimizes injury to the heart tissue and risk to the patient while still causing the cellular damage believed necessary for revascularization of ischemic tissue.
In one embodiment, an ultrasonic device is used to form localized, precisely placed thermal lesions in and near the ischemic tissue. Ultrasonic energy can be advantageously locally directed and is relatively easy to control. Therefore, the thermal lesions formed by ultrasonic energy can induce angiogenesis in myocardium without creating channels and without excessive damage and trauma to the tissue. Even beyond inducing angiogenesis, it is believed that the method will assist in increasing blood flow to the treated region and mitigates the progression and symptoms of ischemia.
In one particular embodiment, a catheter having a distal end is inserted into the patient. The catheter has at least one ultrasonic transducer on the distal end. The ultrasonic transducer is positioned proximate to the ischemic region. Ultrasonic energy is applied at a frequency at or above 1 MHz to create a first thermal lesion in the ischemic region of the myocardium. For example, the ultrasonic energy can be applied at frequencies between 4 MHz and 15 MHz to create the thermal lesion.
The method may further include repositioning the ultrasonic transducer and applying ultrasonic energy at a frequency at or above 1 MHz to create one or more second thermal lesions in the myocardium. The second thermal lesion(s) may be created in the ischemic region adjacent the first thermal lesion or in myocardium adjacent the ischemic region. The first and second lesions can be created so as to have a gradient of sizes.
An embodiment of an ultrasonic catheter within the present invention can have an array of ultrasonic transducers. Ultrasonic energy at a frequency greater than approximately 1 MHz can be applied from the ultrasonic transducers in the array to create additional thermal lesions. The ultrasonic transducers of the array may be independently coupled to a power source and independently controlled by a controller, allowing ultrasonic energy of varying power and duration to be independently applied from each transducer in the array. The controller can control the duty cycle of the power source, so that higher powers can be applied to the tissue without overheating the transducer. The independently controlled transducers in the array can advantageously be used to create multiple thermal lesions that have a gradient of sizes.
The ultrasonic transducer can have a shape that causes the ultrasonic energy emitted by the transducer to converge in a region located internal to the myocardium and at a distance from the endocardium and epicardium. The transducer can have, for example, a bowl-like, partial cylinder, or hollow hyperboloid-like shape. Accordingly, the thermal lesions produced by the converging ultrasonic energy will be located internal to the myocardium and distal from the endocardium and epicardium. This allows ischemic region internal to the myocardium to be treated without injuring the endocardium or epicardium.